Java Package, Interface & Exception Package Prepared By - Rifat Shahriyar 2 Package • Java package provides a mechanism for partitioning the class name space into more manageable chunks – Both naming and visibility control mechanism • Define classes inside a package that are not accessible by code outside that package • Define class members that are exposed only to other members of the same package • This allows classes to have intimate knowledge of each other – Not expose that knowledge to the rest of the world Prepared By - Rifat Shahriyar 3 Declaring Package • package pkg – Here, pkg is the name of the package • package mypackage – creates a package called mypackage • The package statement defines a name space in which classes are stored • If you omit the package statement, the class names are put into the default package, which has no name Prepared By - Rifat Shahriyar 4 Declaring Package • Java uses file system directories to store packages – the .class files for any classes that are part of mypackage must be stored in a directory called mypackage • More than one file can include the same package statement • The package statement simply specifies to which package the classes defined in a file belong • To create hierarchy of packages, separate each package name from the one above it by use of a (.) Prepared By - Rifat Shahriyar 5 Package Example javac -d . AccountBalance.java java mypackage.AccountBalance Prepared By - Rifat Shahriyar 6 Package Syntax • The general form of a multilevel package statement – package pkg1[.pkg2[.pkg3]] – package java.util.concurrent • import statements occur immediately following the package statement and before any class definitions • The general form of the import statement – import pkg1 [.pkg2].(classname | *) – import java.util.Scanner – import statement is optional, class can be used with name that includes full package hierarchy Prepared By - Rifat Shahriyar 7 Access Protection • Packages act as containers for classes and other subordinate packages • Classes act as containers for data and code • The class is Java’s smallest unit of abstraction • Four categories of visibility for class members – Subclasses in the same package – Non-subclasses in the same package – Subclasses in different package – Classes that are neither in the same package nor subclasses Prepared By - Rifat Shahriyar 8 Access Protection • The three access modifiers provide a variety of ways to produce the many levels of access required – private, public, and protected • The following applies only to members of classes Private No Modifier Protected Public Same class Yes Yes Yes Yes Same package subclass No Yes Yes Yes Same package non-subclass No Yes Yes Yes Different package subclass No No Yes Yes Different package non-subclass No No No Yes For detail example, please refer to codes in package p1 and p2 Prepared By - Rifat Shahriyar 9 Access Protection • Anything declared public can be accessed from anywhere • Anything declared private cannot be seen outside of its class • When a member does not have an explicit access specification, it is visible to subclasses as well as to other classes in the same package (default access) • If you want to allow an element to be seen outside your current package, but only to classes that subclass the class directly, declare that protected Prepared By - Rifat Shahriyar 10 Access Protection • A non-nested class has only two possible access levels – default and public (others are abstract and final) • When a class is declared as public, it is accessible by any other code • If a class has default access, then it can only be accessed by other code within its same package • When a class is public, it must be the only public class declared in the file, and the file must have the same name as the class Prepared By - Rifat Shahriyar 11 Interface Prepared By - Rifat Shahriyar 12 Interface • We can call it a pure abstract class having no concrete methods – All methods declared in an interface are implicitly public and abstract – All variables declared in an interface are implicitly public, static and final • An interface can’t have instance variables, so can’t maintain state information unlike class • A class can only extend from a single class, but a class can implement multiple interfaces Prepared By - Rifat Shahriyar 13 Implementing Interface • When you implement an interface method, it must be declared as public • By implementing an interface, a class signs a contract with the compiler that it will definitely provide implementation of all the methods – If it fails to do so, the class will be considered as abstract – Then it must be declared as abstract and no object of that class can be created • An abstract class specifies what an object is and an interface specifies what the object can do Prepared By - Rifat Shahriyar 14 Simple Interface Prepared By - Rifat Shahriyar 15 Simple Interface Prepared By - Rifat Shahriyar 16 Applying Interfaces Prepared By - Rifat Shahriyar 17 Nested or Member Interfaces Prepared By - Rifat Shahriyar 18 Variables in Interfaces Prepared By - Rifat Shahriyar 19 Extending Interfaces Prepared By - Rifat Shahriyar 20 Default Interface Methods • Prior to Java 8, an interface could not define any implementation whatsoever • The release of Java 8 has changed this by adding a new capability to interface called the default method – A default method lets you define a default implementation for an interface method – Its primary motivation was to provide a means by which interfaces could be expanded without breaking existing code Prepared By - Rifat Shahriyar 21 Default Interface Methods Prepared By - Rifat Shahriyar 22 Multiple Inheritance Issues Prepared By - Rifat Shahriyar 23 Static Methods in Interface Prepared By - Rifat Shahriyar 24 Private Methods in Interface Prepared By - Rifat Shahriyar 25 Exception Prepared By - Rifat Shahriyar 26 Exception Handling • When an exceptional condition arises, an object representing that exception is created and thrown in the method that caused the error – That method may choose to handle the exception or pass it on (caught and processed at some point) • Generated by the Java runtime or by your code – Exceptions thrown by Java relate to fundamental errors that violate the rules of the Java language or the constraints of the Java execution environment – Manually generated exceptions are typically used to report some error condition to the caller of a method Prepared By - Rifat Shahriyar 27 Exception Handling • Java exception handling is managed via five keywords – Program statements that you want to monitor for exceptions are contained within a try block – If an exception occurs within the try block, it is thrown – Your code can catch this exception (using catch) – To manually throw an exception, use the keyword throw – Any exception that is thrown out of a method must be specified as such by a throws clause – Any code that absolutely must be executed after a try block completes is put in a finally block Prepared By - Rifat Shahriyar 28 Exception Classes Hierarchy Image Source: https://simplesnippets.tech/exception-handling-in-java-part-1/ Complete List of Java Exceptions: https://programming.guide/java/list-of-java-exceptions.html Prepared By - Rifat Shahriyar 29 Uncaught Exceptions Prepared By - Rifat Shahriyar 30 Caught Exceptions Prepared By - Rifat Shahriyar 31 Caught Exceptions try can be nested, please refer to ExceptionTryNested.java Prepared By - Rifat Shahriyar 32 finally Prepared By - Rifat Shahriyar 33 Multiple catch clauses catch(ArithmeticException | NullPointerException e) catch(ArithmeticException | Exception e) - Error Prepared By - Rifat Shahriyar 34 throw Prepared By - Rifat Shahriyar 35 throws throws listing is not required for those of RuntimeException or any of their subclasses Prepared By - Rifat Shahriyar 36 Custom Exceptions Prepared By - Rifat Shahriyar 37.